1. Field of the Invention
The present invention relates generally to a thermal synthesis process. In particular, the present invention relates to methods and apparatus for thermal conversion of reactants in a thermodynamically stable high temperature gaseous stream to desired end products, such as either a gas or ultrafine solid particles.
2. Relevant Technology
Natural gas (where methane is the main hydrocarbon) is a low value and underutilized energy resource in the United State. Huge reserves of natural gas are known to exist in remote areas of the continental U.S., but this energy resource cannot be transported economically and safely from those regions. Conversion of natural gas to higher value hydrocarbons has been researched for decades with limited success in today""s economy. Recently, there have been efforts to evaluate technologies for the conversion of natural gas (which is being flared) to acetylene as a feed stock for commodity chemicals. The ready availability of large natural gas reserves associated with oil fields and cheap labor might make the natural gas to acetylene route for producing commodity chemicals particularly attractive in this part of the world.
Acetylene can be used as a feed stock for plastic manufacture or for conversion by demonstrated catalyzed reactions to liquid hydrocarbon fuels. The versatility of acetylene as a starting raw material is well known and recognized. Current feed stocks for plastics are derived from petrochemical based raw materials. Supplies from domestic and foreign oil reserves to produce these petrochemical based raw materials are declining, which puts pressure on the search for alternatives to the petrochemical based feed stock. Therefore, the interest in acetylene based feed stock has currently been rejuvenated.
Thermal conversion of methane to liquid hydrocarbons involves indirect or direct processes. The conventional methanol-to-gasoline (MTG) and the Fischer-Tropsch (FT) processes are two prime examples of such indirect conversion processes which involve reforming methane to synthesis gas before converting to the final products. These costly endothermic processes are operated at high temperatures and high pressures.
The search for direct catalytic conversion of methane to light olefins (e.g., C2H4) and then to liquid hydrocarbons has become a recent focal point of natural gas conversion technology. Oxidative coupling, oxyhydrochlorination, and partial oxidation are examples of direct conversion methods. These technologies require operation under elevated pressures, moderate temperatures, and the use of catalysts. Development of special catalysts for direct natural gas conversion process is the biggest challenge for the advancement of these technologies. The conversion yields of such processes are low, implementing them is costly in comparison to indirect processes, and the technologies have not been proven.
Light olefins can be formed by very high temperature ( greater than 1800xc2x0 C.) abstraction of hydrogen from methane, followed by coupling of hydrocarbon radicals. High temperature conversion of methane to acetylene by the reaction 2CH4xe2x86x92C2H2+3H2 is an example. Such processes have existed for a long time.
Methane to acetylene conversion processes currently use cold liquid hydrocarbon quenchants to prevent back reactions. Perhaps the best known of these is the Huels process which has been in commercial use in Germany for many years. The electric arc reactor of Huels transfers electrical energy by xe2x80x98directxe2x80x99 contact between the high-temperature arc (15000-20000 K) and the methane feed stock. The product gas is quenched with water and liquefied propane to prevent back reactions. Single pass yields of acetylene are less than 40% for the Huels process. Overall C2H2 yields are increased to 58% by recycling all of the hydrocarbons except acetylene and ethylene.
Although in commercial use, the Huels process is only marginally economical because of the relatively low single pass efficiencies and the need to separate product gases from quench gases. Subsidies by the German Government have helped to keep this process in production.
A similar process with 9 MW reactors was built by DuPont and operated between 1963 and 1968 supplying acetylene produced from liquid hydrocarbon sources to a neoprene plant. The process was also reportedly demonstrated at the pilot-plant scale using methane feed. The plant-scale operation was limited to liquefied petroleum gas or liquid hydrocarbon distillates. The size of the DuPont pilot scale process is not reported. In the DuPont process the arc was magnetically rotated while in the original Huels process the arc is xe2x80x9cswirl stabilizedxe2x80x9d by tangential injection of gases. In the DuPont process, all feedstock, diluted with hydrogen, passed through the arc column. In the Huels process, a fraction of the reactants are injected downstream of the arc.
Westinghouse has employed a hydrogen plasma reactor for the cracking of natural gas to produce acetylene. In the plasma reactor, hydrogen is fed into the arc zone and heated to a plasma state. The exiting stream of hot H2 plasma at temperatures above 5000 K is mixed rapidly with the natural gas below the arc zone, and the electrical energy is indirectly transferred to the feed stock. The hot product gas is quenched with liquefied propane and water, as in the Huels process, to prevent back reactions. However, as with the Huels process, separation of the product gas from quench gas is needed. Recycling all of the hydrocarbons except acetylene and ethylene has reportedly increased the overall yield to 67%. The H2 plasma process for natural gas conversion has been extensively tested on a bench scale, but further development and demonstration on a pilot scale is required.
The Scientific and Industrial Research Foundation of Norway has developed a reactor consisting of concentric, resistance-heated graphite tubes. Reaction cracking of the methane occurs in the narrow annular space between the tubes where the temperature is 1900 to 2100 K. In operation, carbon formation in the annulus led to significant operational problems. Again, liquefied quenchant is used to quench the reaction products and prevent back reactions. As with the previous two acetylene production processes described above, separation of the product gas from quench gas is needed. The overall multiple-pass acetylene yield from the resistance-heated reactor is about 80% and the process has been tested to pilot plant levels.
Accordingly, it is desirable to improve upon the modest methane conversion efficiencies, acetylene yields, selectivities, and specific energy requirements observed in the above processes.
Titanium""s properties of high corrosion resistance and strength, combined with its relatively low density, result in titanium alloys being ideally suited to many high technology applications, particularly in aerospace systems. Applications of titanium in chemical and power plants are also attractive.
Unfortunately, the widespread use of titanium has been severely limited by its high cost. The magnitude of this cost is a direct consequence of the batch nature of the conventional Kroll and Hunter processes for metal production, as well as the high energy consumption rates required by their usage.
The large scale production processes used in the titanium industry have been relatively unchanged for many years. They involve the following essential steps: (1) Chlorination of impure oxide ore, (2) purification of TiCl4 (3) reduction by sodium or magnesium to produce titanium sponge, (4) removal of sponge, and (5) leaching, distillation and vacuum remelting to remove Cl, Na, and Mg impurities. The combined effects of the inherent costs of such processes, the difficulty associated with forging and machining titanium and, in recent years, a shortfall in sponge availability, have contributed to relatively low titanium utilization.
One of the most promising techniques currently undergoing development to circumvent the high cost of titanium alloy parts is powder metallurgy for near net shape fabrication. For instance, it has been estimated that for every kilogram of titanium presently utilized in an aircraft, 8 kilograms of scrap are created. Powder metallurgy can substantially improve this ratio. Although this technology essentially involves the simple steps of powder production followed by compaction into a solid article, considerable development is currently underway to optimize the process such that the final product possesses at least equal properties and lower cost than wrought or cast material.
One potential powder metallurgy route to titanium alloy parts involves direct blending of elemental metal powders before compaction. Presently, titanium sponge fines from the Kroll process are used, but a major drawback is their high residual impurity content (principally chlorides), which results in porosity in the final material. The other powder metallurgy alternative involves direct use of titanium alloy powder subjected to hot isostatic pressing.
Several programs are currently involved in the optimization of such titanium alloy powders. Results are highly promising, but all involve Kroll titanium as a starting material. Use of such existing powders involves a number of expensive purification and alloying steps as outlined above.
The formation of titanium under plasma conditions has received intermittent attention in the literature over the last 30 years. Reports have generally been concerned with the hydrogen reduction of titanium tetrachloride or titanium dioxide with some isolated references to sodium or magnesium reduction.
The use of hydrogen for reducing titanium tetrachloride has been studied in an arc furnace. Only partial reduction took place at 2100 K. The same reaction system has been more extensively studied in a plasma flame and patented for the production of titanium subchloride (German Patent 1,142,159, Jan. 10, 1963) and titanium metal (Japanese Patents 6854, May 23, 1963; 7408, Oct. 15, 1955; U.S. Pat. No. 3,123,464, Mar. 3, 1964).
Although early thermodynamic calculations indicated that the reduction of titanium tetrachloride to metallic titanium by hydrogen could start at 2500 K, the system is not a simple one. Calculations show that the formation of titanium subchloride would be thermodynamically more favorable in that temperature region.
U.S. Pat. No. 3,123,464, discloses that reduction of titanium tetrachloride to liquid titanium can be successfully carried out by heating the reactants (TiCl4 and H2) at least to, and preferably in excess of, the boiling point of titanium (3535 K). At such a high temperature, it was disclosed that while titanium tetrachloride vapor is effectively reduced by atomic hydrogen, the tendency of H2 to dissolve in or react with Ti is insignificant, the HCl formed is only about 10% dissociated, and the formation of titanium subchlorides could be much less favorable. The titanium vapor product is then either condensed to liquid in a water-cooled steel condenser at about 3000 K, from which it overflows into a mold, or is flash-cooled by hydrogen to powder, which is collected in a bin. Since the liquid titanium was condensed from gas with only gaseous by-products or impurities, its purity, except for hydrogen, was expected to be high.
Japanese Patent 7408, describes reaction conditions as follows: a mixture of TiCl4 gas and H2 (50% in excess) is led through a 5 mm inside diameter nozzle of a tungsten electrode at a rate of 4xc3x9710xe2x88x923m3/min and an electric discharge (3720 V and 533 mA) made to another electrode at a distance of 15 mm. The resulting powdery crystals are heated in vacuo to produce 99.4% pure titanium.
In neither of the above patents is the energy consumption clearly mentioned. Attempts to develop the hydrogen reduction process on an industrial scale were made using a skull-melting furnace, but the effort was discontinued. More recently, a claim was made that a small quantity of titanium had been produced in a hydrogen plasma, but this was later retracted when the product was truly identified as titanium carbide.
In summary, the history of attempts to treat TiCl4 in hydrogen plasmas appears to indicate that only partial reduction, i.e., to a mixture of titanium and its subchlorides, is possible unless very high temperatures ( greater than 4000 K) are reached. Prior researchers have concluded that extremely rapid, preferential condensation of vapor phase titanium would be required in order to overcome the unfavorable thermodynamics of the system.
Accordingly, there is a need for methods and apparatus that overcome or avoid the above problems and limitations.
It is an object of the invention to provide a method and apparatus for thermal conversion of one or more reactants in a thermodynamically stable high temperature gaseous stream to a desired end product.
It is a further object of the invention to provide an improved process conversion efficiency and product yield in the thermal conversion of one or more reactants in a thermodynamically stable high temperature gaseous stream to a desired end product.
It is yet another object of the invention to provide a method and apparatus for increasing acetylene yield from the thermal conversion of natural gas.
To achieve the foregoing objects, and in accordance with the invention as embodied and broadly described herein, a method and apparatus are provided for thermal conversion of one or more reactants in a thermodynamically stable high temperature gaseous stream to a desired end product in the form of a gas or ultrafine solid particles. In general, the method includes the following steps. First, a reactant stream is introduced at one end of an axial reactor. Next, the reactant stream is heated as the reactant stream flows axially through an injection line having a reduced diameter with respect to the axial reactor to produce turbulent flow and thereby thoroughly mix the reactant stream with a heating gas. Thereafter, the thoroughly mixed reactant stream is passed axially through a reaction zone of the axial reactor, with the reaction zone maintained at a substantially uniform temperature over the length of the reaction zone. The axial reactor has a length and a temperature and is operated under conditions sufficient to effect heating of the reactant stream to a selected reaction temperature at which a desired product stream is produced at a location adjacent the outlet end of the axial reactor.
In particular, the method of the invention comprises the following steps. First, a stream of plasma arc gas is introduced between the electrodes of a plasma torch including at least one pair of electrodes positioned adjacent to an inlet end of an axial reactor chamber. The stream of plasma arc gas is introduced at a selected plasma gas flow while the electrodes are subjected to a selected plasma input power level to produce a plasma in a restricted diameter injection line that extends into the reactor chamber and toward an outlet end of the reactor chamber. Second, an incoming reactant stream is thoroughly mixed into the plasma by injecting at least one reactant into the injection line to produce the thorough mixing prior to introduction into the reactor chamber. The reactor chamber is maintained at a substantially uniform temperature over the flow field for the reactions to reach equilibrium. The gaseous stream exiting a nozzle is cooled at the outlet end of the reactor chamber by reducing its velocity while removing heat energy at a rate sufficient to prevent increases in its kinetic temperature. The desired end products are then separated from the gases remaining in the cooled gaseous stream.
The present invention provides improvements in process conversion efficiency and acetylene yield over prior conventional processes. Such improvements are primarily accomplished by more efficient injection and mixing of reactants with plasma gases, and minimization of temperature gradients and cold boundary layers in the reactor. The improved mixing and thermal control also leads to increased specificity reducing the yield of hydrocarbons other than acetylene. The quench rate achieved by wall heat transfer in small reactors is adequate to arrest acetylene decomposition and soot formation. Formation of other hydrocarbon species, except for ethylene, is unaffected by significantly increasing the quench rate.
These and other objects, features, and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.